build any new pc around the i5 3570k cpu and you cant really go wrong, get 8+gb of ram, 650+ geforce card (660ti+ if he wants hardcore gaming), 120gb ssd, 600w+ power supply (dont skimp on this buy a good brand) and a case with decent cooling, win7 64 bit and off ya go, should be around 6-700 without the monitor and for that I would heartily recommend anything 120hz+ for gaming